1. Field of Invention
The present invention pertains to controlling laser power for effectively identifying and monitoring concentrations of the constituents or species within a gas stream under harsh and/or other processing conditions.
2. Related Art
Optical fiber amplifier technology has historically focused on communication applications. The use of optical amplifiers in this technology allows a communication signal to remain in the optical domain of the transmission link without changing back to the electrical domain as experienced by opto-electronic signal regenerators. In the early signal transmissions systems, optical signal attenuation resulting from losses in the fiber was recovered by a repeater, which converts the optical signal into an electrical signal that can be reshaped, retimed, and regenerated back to an optical signal for continued transmission. The drawback of the electronic regenerative repeaters is the need for high-speed electronics that becomes increasingly complex and problematic in terms of equipment size and bit rate transmission. These problems were overcome by implementing an active fiber amplifier that reshapes the transmission signal completely in the optical domain.
Optical fiber amplifier technology is well known in the art, and the theory of operation, characteristics, and applications of this technology has been described in Sudo Shoichi, Optical Fiber Amplifiers: Materials, Devices, and Applications, Artech House, Inc., 1997. Briefly, the operating principle of the fiber amplifier is based on using a rare-earth doped section of fiber to amplify the entering optical signal using stimulated emission of the optically excited rare earth ions in the fiber core that are pumped by a semiconductor laser. The amplified signal exiting the fiber can be several orders of magnitude higher than the input signal with minimal addition of intensity noise (typically less than 7 dB). A summary of all the rare-earth ion energy levels and laser transitions that have been reported and that span a wavelength range from 0.172-5.15 μm is described in Caird, J. A. and Payne, S. A., Crystal Paramagnetic Ion Lasers, CRC Handbook of Laser Science and Technology, Supplement 1: Lasers, Weber, M. J., (ed.), Boca Raton Fla.: CRC Press, pp 3-100, 1991. The transitions lying between 0.8 μm and 1.6 μm are of particular importance in applications where fiber optic lines exhibit low losses (e.g., telecommunication applications). In this region, the rare earth dopant fiber amplifiers such as erbium-doped fiber amplifier (EDFA), praseodymium-doped fiber amplifier (PDFA), thulium-doped fiber amplifier (TDFA), neodymium-doped fiber amplifier (NDFA), and ytterbium-doped fiber amplifier (YDFA) are used. The amplification bands and bandwidth are dependent on the different kinds of dopants used in the fiber composition. The form of the bandwidth can also be influenced by utilizing co-dopants and host glass material. Of the different rare earth dopant amplifiers, EDFA's have received the greatest attention due to the low fiber attenuation loses in the C (1530 nm-1565 nm) and L (1565 nm-1605 nm) bands that are used for telecommunication applications.
An energy diagram for fiber amplification is depicted in FIG. 1 based upon the selection of the rare-earth element erbium and 1.5 μm light amplification. In the 1.5 μm region, a 980 nm pump laser can be used to pump the 4I11/2 level. Population of the 4I13/2 level occurs via fast nonradiative decay with vibrational phonons of the crystal lattice. Subsequently, signal photons near 1.5 μm are multiplied through stimulated emission from the 4I13/2 level to the ground state. While it is noted that the pump laser choice is not restricted to 980 nm, an EDFA based on a 980 nm pump laser has favorable noise characteristics. There are multiple choices for the pump laser, but for practical EDFA's, 980 nm and 1480 nm pumping are mainly used due to their pump efficiency and high-output power characteristics.
For commercially available fiber amplifiers (FA), three standard modes of operation are used: automatic power control (APC), automatic gain control (AGC) and automatic current control (ACC). APC mode takes a calibrated power reading from an optical tap on the output. This measured value is used as feedback for a control loop which adjusts the pump-laser injection current to keep the output power constant. AGC mode is similar except taps are located in the input and output portions of the amplifier and the injection current is automatically adjusted to maintain a fixed ratio. ACC mode includes two types of control loops, the first type of control loop monitors the pump laser output power and adjusts the injection current to keep pump power constant, while the second type of control loop monitors and controls the injection current independent of the output power.
In addition to telecommunication applications, fiber amplifiers are also utilized in systems for facilitating spectroscopy measurements of molecules, where the amplifier is used for nonlinear wavelength mixing applications and photoacoustic spectroscopy. Application of near-IR laser systems for in-situ process monitoring relies on the ability to transmit a laser beam or signal through the process and receive sufficient radiation at the detection side for monitoring gas species concentration and/or gas temperature. On processes with high particle densities, the transmission of light can be severally affected requiring a measurement pathlength reduction and/or increasing the laser power. However, available diode lasers used in the near-IR region have limited power output. Overcoming the power limitation can be obtained using a fiber amplifier to extend the power range of the laser by several orders of magnitude for improved transmission and background discrimination.
For example, it is known to implement a Yb fiber amplifier to generate 600 mW at 1083 nm from an input laser power of about 15 mW for difference frequency mixing (DFM) to produce tunable radiation in the mid-IR range between 3-5 μm. See, e.g., Richter, Dirk, Lancaster, David, G., and Tittel, Frank, K., Development of an Automated Diode-laser-based Multicomponent Gas Sensor, Applied Optics, Vol. 39, No. 24, August 2000 and Lancaster, D. G., Richter, D., and Tittel, F. K., Portable Fiber-coupled Diode-laser-based Sensor for Multiple Trace Gas Detection, Applied Physics B. Vol. 69, 1999. In these applications, two near-IR diode lasers are mixed in a nonlinear optical material such as periodic poled lithium niobate (PPLN) to generate narrow linewidth tunable mid-IR radiation. Access to the mid-IR radiation range has applications for trace gas sensing constituents or species such as CO2, NO2, CH4, N2O, HCl, H2CO, CH3OH, and C6H6. Implementing a fiber amplifier into the optical layout is beneficial since the resulting output power levels generated from DFM are dependent on the input power used with the nonlinear optical material. Thus, the use of μW power levels in the mid-IR range are possible in these systems by utilizing the increased power boost provided by the Yb fiber amplifier.
As described in Ray, G. J., Anderson, T. N., Lucht, R. P., Walther, T., and Caton, J. A., Fiber-Amplified, Diode-Laser-Based Sensor for OH Absorption, Paper No. 268, Second Joint Meeting of the U.S. Sections of the Combustion Institute, Oakland, Calif., Mar. 25-28, 2001, an Nd-doped fiber amplifier is used for second harmonic generation of 532 nm light from a PPLN crystal that was further frequency doubled with BBO (beta barium borate) to generate 266 nm for OH absorption measurements in a hydrogen-air flame. Here, an ECDL (external cavity diode laser) was utilized to produce 20 mW of the fundamental 1064 nm that was amplified to 1 W using a NDFA. As in the previously described DFM system, higher input laser power for these nonlinear mixing processes produces higher output powers that are useable for practical measurement systems. An advantage for using a fiber amplifier with the nonlinear mixing techniques such as DFM and SHG is an instrumentation size reduction and reduced complexity by using standard diode lasers and fiber optic components.
It is known to utilize a fiber amplifier in a laser powered gas monitoring system to facilitate the direct measurement of a chemical species, as described in Weber, Micheal, E., Pushkarsky, Michael, B., and Patel, Kumar, N., Ultra-Sensitive Gas Detection Using Diode Lasers and Resonant Photoacoustic Spectroscopy, SPIE's International Symposium and Technology, Diode Lasers and Applications, Paper Number 4871-11, July 2002. In this system, an EDFA is used for CO2 and NH3 detection by photoacoustic spectroscopy. This system employs a technique where a laser beam passes through a sample cell containing a gas species having an absorption transition within the tuning range of the laser. As the laser wavelength is tuned to an absorption transition of the molecule the upper energy level is populated. By collision with other atoms or molecules in the cell, these excited molecules transfer their excitation energy completely or partly with collision partners. This energy transfer process results in a rise of temperature and pressure at a constant density in the sample cell. By sweeping the laser wavelength periodically across the molecule absorption transition, periodic pressure variations in the cell emerge, and these pressure variations can be detected with a sensitive microphone. The resulting signal strength observed is directly proportional to the incident laser power.
In U.S. Pat. No. 6,252,689, a distributed fiber network system is described for conducting measurements at multiple locations of an external condition, such as humidity, temperature, gas, pressure, object location, people detection, air velocity, and displacement. Here, the use of a fiber amplifier incorporated into the distributed fiber network is used to boost laser power that has been attenuated by the number of switches, hubs, open path cells, etc., and to act as a repeater for boosting returning optical signals that have undergone attenuation. For this monitoring concept, both bi-directional and unidirectional operation is proposed with the light signal affected by the specific measurement (external condition) returned by fiber optic to the centralized detection system. Insertion losses for each of these components in the network can influence the measurement capability. This system is designed for ambient monitoring and uses a centralized light source, laser or LED, and detection system with the light distributed to multiple locations through a network of switches and hubs. However, the '689 patent does not disclose utilizing the fiber amplifier for transmission improvements due to loses by particulate matter or controlling the laser power at different measurement locations.
The use of a fiber amplifier in a system for directly monitoring gas species is described in U.S. Patent Application Publication No. 2003/0132389, the disclosure of which is incorporated herein by reference in its entirety. In this system, several variations are presented for using a diode laser with a fiber amplifier. In the embodiments described in this reference, the control of the laser power delivered to a particular process was performed by adjusting the pump laser power to thereby adjust the amplifier gain.
In applications where control of the laser power over a large dynamic range is required, or when a portion of the laser radiation is used for a reference, the nonlinear behavior of the fiber amplifier can be problematic in carrying out the measurement. In addition to the non-linearity of the FA, other aspects of the FA operating characteristics, such as noise added to the output radiation, are important considerations that place limitations on the gas sensing detection limits. For example, the noise figure (NF) given by the following equation:
                              N          ⁢                                          ⁢          F                =                  10          ⁢                                          ⁢          log          ⁢                                                    (                                  S                  /                  N                                )                            IN                                                      (                                  S                  /                  N                                )                            OUT                                                          (        1        )            where S is the signal and N is the noise, a theoretically best obtainable value is 3 dB, with typical devices performing no better than 4.5 dB. Saitoh, T. and Mukail, T., “1.5 mm GAINASP Teaveling-Wave Semiconductor Laser Amplifier,” IEEE Journal of Quantum Electronics, vol. 23, no. 6, June 1987.
The NF data from the CATV Amplifier User's Manual (provided by Keopsys, Inc. of Hampton, N.J.) for an EDFA is depicted in FIG. 2 as a function of input power for a range of gain settings and when operating at a fixed wavelength of 1560 nm. This figure illustrates the variation for NF at different gain levels, with gain being defined by the following equation:
                    G        =                  10          ⁢                                          ⁢                      log            ⁡                          (                                                P                  out                                                  P                                      i                    ⁢                                                                                  ⁢                    n                                                              )                                                          (        2        )            where Pin is typically referenced at 1 mW laser input laser power yielding units of dBm on the gain. FIG. 2 indicates that, for various gain settings, the resulting NF ranges from below 5 dB to greater than 8 dB. The higher NF values occur at low gain conditions, e.g., conditions with input powers near 10 dbm (10 mW) and output power of 13 dbm (20 mW). Therefore, operating at higher gain is preferred to minimize the NF on the output. Dynamic process monitoring where the gain must be varied due to changes in the transmission will result in poorer signal detection when the gain must be reduced to lower levels to avoid detector saturation. The added noise resulting from the gain change will in turn reduce the measurement quality.
Output power versus input power data at different wavelengths residing in the C-band for the EDFA is depicted in FIG. 3. In particular, FIG. 3 illustrates two main effects on the operation of the EDFA. First, at low input powers the gain is a strong function of the wavelength that can influence the measurement for multiplexed laser systems that span a large wavelength region. For example, when operating the EDFA with a sampled grating-DBR laser that is capable of tuning over the 40 nm C-band wavelength range, or with a multiplexed laser system using several DFB lasers and beam combiners, would result in variations in the output power at different wavelengths. If the application requires monitoring both 1540 nm and 1560 nm wavelengths with a low input power of −6 dBm (0.25 mW), the variation in the output power between the two wavelengths would be 94 mW, which would saturate the detection electronics when monitoring the 1540 nm radiation.
Second, for a fixed wavelength the performance of the amplifier is nonlinear, where linearity is expressed as operation with constant gain. The dashed curve in FIG. 3 illustrates the expected linear performance behavior if achievable at 1560 nm for a constant gain of 20 dBm. Linear performance of such amplifiers is only achieved over a small input range of about −22 dBm to about −18 dbm (6.3 μW-16 μW). At these low input levels, the advantages of using an amplifier are lost, since the resulting output laser powers are between 3 to 8 mW (assuming 27 dBm gain, i.e., 500× amplification for the linear range input powers). Laser powers in this range and greater are easily achieved from standard off-the-shelve telecommunication DFB diode lasers without amplification.